Fluorenyl complexes of zirconium and hafnium as catalysts for olefin polymerization

In the continuously expanding panorama of Group 4 metallocene complexes and the extensive research devoted to their use as olefin polymerization catalysts, fluorenyl complexes of zirconium and hafnium play a very special role. This singularity can be explained in terms of a particularly versatile bonding potential of the fluorenyl ligand causing a facile change from η5 → η3 → η1 (ring slippage) and thus providing new coordination sites in catalytic cycles. The substitution of fluorenyl ligands in unbridged or bridged (ansa) metallocene complexes allows the variation of steric requirements and symmetry of these complexes. Both parameters have a strong influence on the catalytic activity and stereospecifity as soon as these catalyst precursors are activated with cocatalysts like methylalumoxane (MAO). Synthetic methods, X-ray structures and catalyst activities are discussed for several compounds together with updated literature.

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